In recent years, there has been a birth of interest in a class of materials commonly referred to as ferroelectric materials which have found wide use in random access memory applications. In such applications, it has been common to employ ferroelectric capacitors which typically evidence high remanent polarization, small size, low leakage current and low fatigue rate. Accordingly, workers in the art have focused their interest upon the development of suitable techniques for the growth of ferroelectric thin films evidencing optimum characteristics for use in such capacitors.
Among the earliest materials investigated for such applications were the perovskite ferroelectrics such as PbZr.sub.1-x Ti.sub.x O.sub.3, commonly known as PZT. Unfortunately, the techniques employed for the preparation of these materials have resulted in the formation of defects which alter the stoichiometry due to the creation of vacancies. As a result of these difficulties, degradation problems such as fatigue, aging and leakage currents which adversely affect the device lifetime often occur.
More recently, workers in the art focused their interest upon the preparation of SrBi.sub.2 Ta.sub.2 O.sub.9 (SBT) films using metallorganic vapor deposition techniques. Thus, for example, Desu et al. in U.S. Pat. No. 5,527,567 disclosed a method for the deposition by chemical vapor deposition techniques of high quality layered structured oxide ferroelectric thin films. These films were deposited at temperatures ranging from 450-800.degree. C. The patentees specifically noted that at temperatures greater than 650.degree. C. poor quality films were produced whereas at temperatures less than 600.degree. C. excellent quality films were obtained. However, the use of the lower temperatures was found to lower the deposition rate, thereby necessitating a two step deposition with a short term deposition at temperatures ranging from 450-600.degree. C. and a longer term deposition at temperatures ranging from 600-700.degree. C.
Desu et al. further discovered that in the one step deposition procedure using high temperatures, heterogeneous nucleation and grain growth frequently occurred on the polycrystalline material employed as the bottom electrode in the capacitor of interest. The films so produced evidenced a non-uniform crystalline structure having a rough surface morphology. The patentees found, however, that in the two step deposition process, the first step yielded a thin uniform nucleation layer with grain growth occurring at the top of the substrate which provided a homogeneous nucleation and grain growth condition for the second deposition step.
Although this prior art technique and related techniques described by workers in the art have enjoyed a limited level of success, they have not proven satisfactory from the standpoint of replication and the attainment of spatially homogeneous incorporation of all cation species. In fact, there has been growing evidence that repeatable, uniform incorporation of bismuth in the growing SBT film is a challenge. Thus, for example, the following phenomena have been noted in efforts to achieve stoichiometric SBT with a smooth, homogeneous morphology.
It has been observed that bismuth incorporation at low pressures is characterized by a highly non-linear relationship between percentage bismuth in the gas phase (relative to other cation precursors) and percentage bismuth in the deposited film. At low and moderate bismuth percentages in the gas phase (relative to strontium and tantalum) it has been found that the incorporation efficiency of bismuth is very low. In fact, it has been found that in order to attain bismuth percentages in the deposited film ranging from 5-20%, the bismuth percentage in the gas must be greater than 82%. Under those circumstances, it has been observed that large amounts of bismuth evidencing unsatisfactory morphology and the presence of large crystallites separated by bismuth poor regions are present in the deposited film. Intermediate percentages, for example, 40-50%, of bismuth in the deposited film at low pressures evidence poor repeatability and the bismuth incorporation is typically microscopically non-uniform.
At low pressures ranging from 1 to 10 torr, bismuth incorporation is found to increase as a result of longer residence times in the flow system. Thus, for a fixed reactor geometry, residence time is increased by increasing pressure or reducing total flow. If the results are plotted as a function of partial pressure of bismuth, then bismuth efficiencies of differing bismuth percentages in the gas phase mesh satisfactorily. At lower gas flows (500-1000 sccm), the efficiency of bismuth scales well with the partial pressure of bismuth. At higher flow rates (2,000 sccm), the efficiencies of bismuth, strontium and tantalum are all higher.
And lastly, the deposition rate of bismuth is found to be nearly independent of time from 0.5 torr to 8 torr. Early growth stages appear as islands which coalesce at longer times. Additionally, it is noted that the growth rate is significantly higher than those at equivalent partial pressures of bismuth for SBT when strontium and tantalum are present.
Studies have also revealed that when SBT is deposited on bare platinum using triphenyl bismuth as a precursor, it is likely that the bismuth incorporates due to the catalytic effect of the platinum which is dependent on the nature of the platinum surface. Upon formation of a continuous SBT oxide film, the nucleation conditions change and bismuth incorporation would appear to be suppressed except on some bismuth oxide crystallites which also may have some catalytic activity. It is apparent that bismuth nucleates and continues to do so at such sites. If bismuth does not nucleate at that site, it will not nucleate since the strontium tantalate film is able to nucleate and grow and this oxide does not favor subsequent bismuth nucleation. Accordingly, bismuth incorporated in the film is highly dependent upon the formation of these crystallites on the platinum surface. Another explanation for this behavior is the presence of a leaving mechanism for bismuth. Bismuth which cannot be built into the film may leave the film either by migration into the substrate or by evaporation.
In light of the foregoing, it is apparent that the attainment of a uniform, high density of nucleation sites for bismuth or other metals used in such processes is essential to attain deposition of films with controllable composition and uniform morphology.